There are various kinds of pressure-sensitive recording sheets including, for example, the combination of an upper sheet having formed on a support a micro-capsule layer, which contains microcapsules of oil drops of an almost colorless electron donating color former dissolved in a solvent, and a lower sheet having an electron accepting developer-containing developer layer formed on another support, and optionally a middle sheet having a microcapsule layer on one side of a support and a developer layer on the other side thereof. Other sheets have the above described microcapsules and developer on the same surface of a support, or either one of the microcapsules or developer incorporated into a support and the other coated thereon.
These pressure-sensitive recording sheets are described, for example, in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,730,457 or 3,418,250.
One important characteristic of pressure-sensitive recording sheets is that the sheets be able to form sharp color images when written or printed under pressure with a ball-point pen or a typewriter, without being color-stained by pressure during manufacture, processing, transportation or storage. In addition, it also is important that the sheets not be stained and the color forming capacity thereof not be lowered, even when they are exposed to water, solvents and heat.
A coacervation method using gelatin has been utilized for microcapsulation of coating compositions for pressure-sensitive recording sheets as described, for example, in U.S. Patent 2,800,457, but recently, various kinds of microcapsulation methods using synthetic polymer materials for walls of microcapsules have been studied and developed. For example, there are a polymerization method for microcapsulation using polyamides, polyurethanes, polyureas, saturated polyesters, etc., as a material for microcapsule walls as described, for example, in British Patent 1,046,409, and an in situ method for microcapsulation using melamine-formaldehyde resins or urea-formaldehyde resins as a material for microcapsule walls as described, for example, in Japanese Patent Publication No. 30282/71.
As a binder for binding the microcapsules thus formed to a support, water-soluble polymers such as polyvinyl alcohol, starch, dextrin, carboxymethyl cellulose, casein, etc., have heretofore been used. The binder is required not to bind microcapsules to a support but also to protect the microcapsules themselves from any external pressure so as to prevent color stain and fog during printing. However, since the above mentioned water-soluble polymers have poor elasticity by themselves, their function of protecting microcapsules is insufficient. Further, since they are poor in the oil swellability, their function of preventing fog during printing is also poor. Thickening the microcapsule walls or increasing the binder amount for the purpose of preventing color stain or fog during printing undesirably reduces the coloring capacity.
Japanese Patent Application (OPI) Nos. 72891/82 and 77589/82 (corresponding to British Patent 2,087,942) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") have proposed adding an acrylate polymer or ethylene-vinyl acetate copolymer in a microcapsule-containing coating composition so as to protect the microcapsule wall. However, the additive is insufficient in adhesive power and hardly prevents fog during printing, and therefore the coloring capacity and the prevention of color stain could not both be satisfied even by the use of the additive.
Japanese Patent Application (OPI) Nos. 97886/85 and 280985/86 have proposed adding a styrene-butadiene copolymer latex to a microcapsule-containing coating composition as an improvement of the above mentioned method. However, the latex essentially requires an ethylenically unsaturated acid monomer to improve the stability of the microcapsule-containing coating composition and the adhesive power of the composition with a support. The electron donating color former in the microcapusles can color because of the acid component, and, therefore, when the latex is used as a binder for microcapsules, the part in which the microcapsules have been broken is color-stained. In addition, if the part is exposed to light, the degree of the color stain is augmented.